Wiresaw cutting is the dominant method for slicing ingots into thin wafers for use in the integrated circuits and photo-voltaics (PV) industries. This method is also commonly used for wafering substrates of other materials, such as sapphire, silicon carbide, or ceramic substrates. A wiresaw typically has a web of fine metal wires, or a wireweb, where the individual wires have a diameter around 0.15 mm and are arranged parallel to each other, at a distance of 0.1 to 1.0 mm, through a series of spools, pulleys and wire guides. Slicing, or cutting, is accomplished by contacting the workpiece (e.g. an ingot) with a moving wire to which an abrasive slurry has been applied.
Conventional wiresaw abrasive slurries typically comprise a carrier and abrasive particles combined by mixing in a ratio of about 1:1 by weight. The carrier is a liquid that provides lubrication and cooling, such as a mineral oil, kerosene, polyethylene glycol, polypropylene glycol or other polyalkylene glycols. The liquid carrier also holds the abrasive to the wire so that the abrasive can contact the workpiece. The abrasive is typically a hard material such as silicon carbide particles.
The wiresaw cutting process is a batch process in which a fixed volume of slurry is continuously recycled from a holding tank and supplied using nozzles to the wireweb close to the workpiece being cut. During the process, the waste material from the cut, including the kerf, gradually accumulates in the slurry, which eventually loses performance. The exhausted slurry is treated and separated into its constituents (i.e. abrasive and carrier). Reclaiming is essential to the economics of the operation.
One component of the spent slurry is magnetic or magnetizable impurities that are generated primarily through wire wear produced during the wiresaw cutting process. Such impurities can interact with components in the slurry and the kerf matter, leading to viscosity changes, pH changes, temperature changes, decomposition of additives, particle size distribution changes or other changes to the slurry. The accumulation of magnetic or magnetizable impurities therefore leads to the degradation of the cutting performance over time.
U.S. Pat. No. 6,264,843 describes a batch process for reclaiming a cutting fluid suspension produced during machining. The described process separates the abrasive containing fraction first and then uses a magnetic separator to remove abraded material. The recovered fluid is then reconstituted with fresh slurry and returned to the cutting process.
U.S. Pat. No. 6,443,143 describes a method of cutting a rare earth alloy using an abrasive free cutting fluid. In the described process kilogram quantities the rare earth alloy kerf matter is removed using centrifugation and a magnetic separator. Because there is no abrasive contained in the spent fluid, the process is designed to remove large quantities of solids.
Although the batch process is acceptable to the industry, changing the slurry slows the wiresaw cutting process and requires more tool start ups that can be stressful on the equipment. Additionally, it is a difficult challenge to remove contaminants from spent cutting fluids containing relatively large quantities of abrasive materials and relatively small quantities of contaminants, without a costly batch process. It would therefore be advantageous to have a wiresaw cutting operation where the contaminant removal is continuous.